This invention relates to thin film magnetic heads.
The structure of magnetic domains within a pole tip is very important in minimizing the amount of noise in the signal and distortion of the signal during readback. "Good" domains are capable of conducting flux by coherent rotation of the magnetization direction in the domains without changing the location of their wall boundaries. FIG. 1 shows domains capable of flux conduction by rotation of the magnetization vector.
As the width of the pole tip W (FIG. 1) is decreased to accommodate narrower track widths, see Fig. 2, the domains that form are "bad" in that they can only conduct flux by wall motion. That is, for the domains to change polarization, the boundary walls move as shown in FIG. 3.
The problem with conduction of flux by wall motion is that the motion of the boundaries can be impeded by imperfections in the material. The boundaries deform and only after the deformation is great enough does the boundary "snap" free of the imperfection. This sudden change in the boundary location is the mechanism which generates Barkhausen noise. Wall motion is a slower process than coherent rotation of magnetization resulting in lower frequency response of the head. This also causes distortion in pulse shape.
Besides depending upon the aspect ratio of the pole tip, what type of domain structure is formed is a function of the composition of the magnetic material of the pole tip and the stress induced in the pole tip during the fabrication process. In fabricating a thin film head magnetic pole, the magnetic material is plated upon a wafer using a mask to form the geometry of the yoke and pole tip. After plating and completion of the wafer processing, the pole tip region is cut and lapped to the desired throat height.
The mask geometry used in the prior art consisted of a large yoke region and a narrower pole tip or neck region. See FIGS. 4a and 4b. The composition of magnetic material in the pole tip in the configuration of FIG. 4 differed significantly from that of the yoke. This difference combined with the stresses created in the material resulted in undesirable magnetic domain structure. It was found, however, that the desired domain structure could be achieved by very precisely controlling the magnetic material composition and the plating field strength followed by annealing the pole. If the parameters varied only slightly, the undesirable pole tip domains would form. The problems in the prior art arose because the current density during the through mask plating was not uniform resulting in non-uniform composition and thickness across the pole geometry.